Hi
If you look at familysearch - South Africa - Church of the Province of South Africa, then click on search all images, then Cape, you will see that they give images for each of the churches. Some are just for Cape Town, and the Church name, and others will be for Cape Town, a suburb or two and the Church name. Unfortunately not all the entries would have marriages for the dates you need.
I would assume that, given that he only arrived late in 1881 they would have had to meet, then go out for a bit before they married. Given that William was born in 1884 I would be looking for a marriage probably round July 1883 - October 1883 bearing in mind that there was no contraception in those days! That could narrow the search down a bit. Obviously if that didn't work then I would go backwards from July.
The other thing you could do would be to look for the baptism of William as the chances would be that he was baptised in the same church as they were married in. The problem there is that children were often baptised quite some time after their birth!
No-one ever said that this family tree business was easy
